1. Field of the Invention
The present invention relates to a method of controlling an actuator for reciprocating a driving member by expansions and contractions of an electromechanical conversion unit, and an image capturing device having such an actuator. The present invention relates more particularly to an image capturing device provided with an image blurring correction function, and a blurring correction method.
2. Description of the Related Art
There has been known a piezoelectric actuator in which a driving section is fixed to an electromechanical conversion unit such as a piezoelectric ceramic, a member of the driving section (a driving member) is reciprocated and displaced by expansion and contraction movements of the electromechanical conversion unit caused by an applied driving signal, and a movable member frictionally connected to the driving member is moved in the direction of the expansion and contraction movements. Such actuator is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-299785.
The principle of driving the above-described piezoelectric actuator will be described with reference to FIGS. 12A to 15.
As shown in FIG. 12A, a left end portion of a piezoelectric ceramic 2 is fixed to a fixing member 1, and a driving shaft (driving member) 3 is fixed to a right end portion of the piezoelectric ceramic 2. This driving shaft 3 is supported by a supporting member (not shown) so as to be movable in an axial direction (lateral direction in FIG. 12). Moreover, a movable member (frictional member) 4 is frictionally connected to the driving shaft 3 (i.e., the movable member 4 does not displace with respect to the driving shaft 3 because of the frictional force between them, but displace when force larger than the frictional force is applied).
On the other hand, a driving signal for expanding and contracting the piezoelectric ceramic 2 is supplied from a driving circuit 5 to an electrode of the piezoelectric ceramic 2.
Next, there will be described a case where the movable member 4 is moved rightwards with reference to FIGS. 12A to 13.
When the movable member 4 is driven rightwards, a sawtooth-shaped driving pulse signal is supplied from the driving circuit 5 to the piezoelectric ceramic 2 as shown in a timing chart of FIG. 13. A change ratio of voltage differs between a rising side (*1) and a falling side (*2) in a waveform of the driving pulse signal. During rapid rising (*1) of the driving pulse signal applied to the piezoelectric ceramic 2, as shown in FIG. 12B, the piezoelectric ceramic 2 shortens leftwards of FIG. 12 rapidly (at high speed). Here, since the speed of the shortening of the piezoelectric ceramic 2 is rather high, an inertial force to hold the movable member 4 in the corresponding position overcomes a frictional connecting force, and the movable member 4 remains in an initial position shown in FIG. 12A.
Next, a moderate falling voltage (*2) of the driving pulse is applied. While the driving pulse signal applied to the piezoelectric ceramic 2 moderately falls in this manner, as shown in FIG. 12C, the piezoelectric ceramic 2 elongates rightwards moderately (at low speed). In this case, as shown in FIG. 12C, the movable member 4 frictionally connected to the driving shaft 3 moves rightwards (direction of arrow A1 in the drawing) together with the driving shaft 3.
Therefore, to move the movable member 4 rightwards by a desired distance (movement amount S), necessary number of the driving pulse signals should be supplied from the driving circuit 5.
Next, there will be described a case where the movable member 4 is moved leftwards with reference to FIGS. 14A to 15.
When the movable member 4 is moved leftwards, a sawtooth-shaped driving pulse signal is supplied from the driving circuit 5 as shown in a timing chart of FIG. 15. The pulse signals shown in FIGS. 15 and 13 have symmetric waveforms with respect to the GND level taken as a base level.
The change ratio of the voltage differs between a falling side (*1′) and a rising side (*2′) of a waveform of the driving pulse signal. During rapid falling (*1′) of the driving pulse signal applied to the piezoelectric ceramic 2, as shown in FIG. 14B, the piezoelectric ceramic 2 elongates rightwards rapidly (at high speed) in the drawing. Here, since the speed of the elongating of the piezoelectric ceramic 2 is rather high, an inertial force to hold the movable member 4 in the corresponding position overcomes a frictional connecting force, and the member remains in an initial position shown in FIG. 14A.
Next, a moderate rising voltage (*2′) of the driving pulse is applied. While the driving pulse signal applied to the piezoelectric ceramic 2 moderately rises in this manner, the piezoelectric ceramic 2 shortens moderately (at low speed). In this case, as shown in FIG. 14C, the movable member 4 frictionally connected to the driving shaft 3 moves leftwards (direction of arrow A2 in the drawing) together with the driving shaft 3.
Therefore, to move the movable member 4 leftwards by a desired distance (movement amount S), necessary number of the driving pulse signals should be supplied from the driving circuit 5.
As described above, when the positive or negative sawtooth-shaped pulse voltages are applied, the movable member 4 can be moved leftwards or rightwards.
In addition, a blurring correction technology is known in which a part of an imaging lens or an imaging unit is displaced depending on vibrations to prevent deterioration of an image in order to prevent the captured image from being deteriorated (blurred) by vibrations applied to an image capturing device such as an electronic camera. As an actuator for displacing the imaging unit depending on the vibrations, there has already existed an image capturing device provided with the piezoelectric actuator constituted as described above.
As described in the principle of the piezoelectric actuator, frictionally connected portions (the driving shaft and the movable member) are disposed in a driving mechanism. When impact exceeding a holding force due to the frictional connection is applied to the image capturing device, the movable member deviates, and the imaging unit connected to the movable member is sometimes displaced to an unintended position. In this case, the actuator cannot be momentarily returned exactly to a position before the impact is applied. Therefore, if the blurring correction is continued, the captured image might be deteriorated more. Moreover, the image is rapidly moved owing to the blurring correction after the impact is applied, so a user might feel a sense of incongruity.
The above-described problems are not limited only to the piezoelectric actuator constituted as shown in FIGS. 12A to 15 but are true for actuators including the frictional connection.